Cast nickel-base alloy



United States Patent 3,459,545 CAST NICKEL-BASE ALLOY Clarence GeorgeBieber, Suifern, and John J. Galka,

Tuxedo, N.Y., assignors to The International Nickel Company, Inc., NewYork, N.Y., a corporation of Delaware No Drawing. Continuation-impart ofapplication Ser. No. 468,154, June 29, 1965. This application Feb. 20,1967, Ser. No. 617,071

Int. Cl. C22c 27/00 US. Cl. 75--171 6 Claims ABSTRACT OF THE DISCLOSUREDirected to a nickel-base alloy particularly useful in the form ofcastings such as in the form of turbine blades The present invention isa continuation-in-part of our copending US. application Ser. No.468,154, filed June 29, 1965, now abandoned.

The present invention is directed to a nickel-base casting alloy whichdevelops high strength at elevated temperatures and is characterized byimproved corrosion resistance and good castability, and, moreparticularly, to a nickel-base casting alloy especially useful for theproduction of vacuum melted and vacuum cast aircraft gas turbine bladesfor use in engines wherein corrosive attack due to oxidation andsulfidation is encountered.

The gas turbine industry has now become firmly established and iscapable of producing gas turbine engines for use in aircraft,automotive, marine and stationary applications which are characterizedby long life and reliability. The requirements imposed upon gas turbineengines by actual and prospective users has placed greater and greaterdemands upon the builders of gas turbines. In turn, the requirements forparts to be used in gas turbine engines has continually been based uponhigher and higher performance standards. A particularly sensitive partin the gas turbine mechanical structure is the turbine blading. It hasbeen found that the temperature requirements imposed upon turbineblading materials is always being raised to a higher level. Furthermore,as performance standards imposed upon the gas turbine engine and theconstituent parts thereof are raised to higher levels, it is found thatother problems are encountered which must be solved. Corrosion of theblading materials due to oxidation, sulfidation and other effects due tothe ambient atmosphere encountered in engine service is one of these.Accordingly, the engine builders have come to expect that the bladingmaterials will not only possess very high strength at temperatures onthe order of 1800 F. and higher but will be resistant to the corrosiveeffects encountered in service, particularly during the long exposuretimes in service now being achieved.

One alloy which has been widely employed with good "ice to oxidation andthermal fatigue, and by good structural stability when exposed to theeffects of temperature and stress for extended periods of time. Thealloy has been used to manufacture cast turbine blades which haveperformed satisfactorily in many hundreds of gas turbine engines. Analloy described in US. application Ser. No. 336,458, now Patent No.3,301,670, and nominally containing about 10% chromium, about 4%molybdenum, about 1% columbium, about 2% tungsten, about 2% tantalum,about 6.5% aluminum, about 1% titanium, about 0.12% carbon, about 0.02%boron, about 0.1% zirconium, and the balance essentially nickel, offersmaterially improved strength properties at temperatures of about 1800 F.than the alloy sold under specification AMS 5391. It is found, however,that the aforementioned alloys have not met more stringent requirementsin regard to corrosion resistance such as sulfidation resistance as morerecently imposed by the aircraft gas turbine manufacturers. It has longbeen postulated that chromium is an element which could usefully beemployed in increased amounts for the purpose of improving corrosionresistance, e.g., sulfidation resistance, of nickelbase high temperaturealloys. However, it is found that when an attempt is made to improve thesulfidation resistance of alloys such as those described hereinbeforemerely by increasing the chromium content thereof that other whollyundesirable effects are encountered. These effects include materiallyreduced elevated temperature strength, increased susceptibility to theformation of undesirable phases during long time exposure to elevatedtemperature, reduced ductility, etc. For example, when the chromiumcontent of an alloy as described in US. application Ser. No. 336,458,now Patent No. 3,301,670, and having the nominal composition set forthhereinbefore is increased from about 10% to about 17%, the life torupture at 1800 F. and 22,000 pounds per square inch (p.s.i.) is reducedfrom about 200 hours to about 23 hours. Such a result is whollyunsatisfactory and is not acceptable, particularly when it is borne inmind that the AMS 5391 alloy described hereinbefore is required to havea rupture life of 30 hours at 1800 F. and 22,000 p.s.i. and regularlyprovides a rupture life of about 50 hours at 1800 F. and 22,000 p.s.i.Accordingly, the art has been presented with the problem of providing anickel-base casting alloy which would have materially improved corrosionresistance, e.g., sulfidation resistance, as compared to the AMS 5391alloy but which would still retain essentially the rupture propertiesdeveloped in the AMS 5391 alloy.

We have now discovered a nickel-base casting alloy having substantialrupture strength at temperatures of the order of up to about 1 800 F.and having improved corrosion resistance, e.g., sulfidation resistance,as compared to prior nickel-base alloys employed for the production ofcast gas turbine blades.

It is an object of the present invention to provide a nickel-basecasting alloy having improved corrosion resistance, e.g., sulfidationresistance.

It is a further object of the invention to provide a nickel base castingalloy characterized by highly useful stressrupture properties attemperatures of the order of up to about 1800 F. and having improvedcorrosion resistance, e.g., sulfidation resistance, as compared to priornickelbase casting alloys when employed for gas turbine bladingpurposes.

Other objects and advantages of the invention will become apparent fromthe following description.

Broadly stated, the present invention comprises a nickelbase alloyhaving high stress-rupture properties and having improved corrosionresistance, e.g., sulfidation resistance, at elevated temperatures, whenproduced as castings in the vacuum melted and vacuum cast form whichcontains about 15%, e.g., 15.5% or 16%, to about 18% chromium, about0.5% to about 2.5% molybdenum, about 0.5% to about 2% columbium, about1% to about 3% tungsten, about 1% to about 3% tantalum, about 6% or 6.5to about 7.5% or about 8% of aluminum plus titanium, up to about 10% orcobalt, about 0.025% to about 0.25% carbon, about 0.01% to about 0.04%or 0.05 boron, about 0.01% to about 0.2%; zirconium, and the balanceessentially nickel.

An essentially cobalt-free alloy contains, by weight, about 16% or 16.5%to about 17.5% chromium, about 1% to about 2% molybdenum, about 0.75% toabout 1.25% columbium, about 1.5% to about 2.5% tungsten, about 1.5% toabout 2.5% tantalum, about 5.5% or 6% to about 6.5% aluminum, up toabout 0.5% titanium, about 0.03% to about 0.07% or 0.08% carbon, about0.015% to about 0.025% boron, about 0.05% to about 0.15% zirconium, upto about 1% cobalt, and the balance essentially nickel. Such alloys arecharacterized by a rupture life at 1800 F. and 22,000 p.s.i. of at leastabout 30 hours or more in combination with improved corrosionresistance, e.g., sulfidation resistance, at elevated temperatures.

It is important that the alloys provided in accordance with theinvention be produced using the purest materials commensurate withreasonable cost. Thus, the contents of subversive impurities such aslead, bismuth, tellurium, sulfur, selenium, phosphorus, oxygen,nitrogen, hydrogen, arsenic, antimony, tin and thallium should be as lowas possible. Cobalt may be present in the alloy in amounts up to about10% or about 15%, e.g., about 4% to 12%, as this element appears tocontribute to the sulfidation resistance of the alloy, although cobaltincreases cost. Iron may be present in impurity amounts, e.g., up toabout 1%, as iron decreases the ability of the alloy to resistmicrostructural changes after long exposure to elevated temperature.Silicon and manganese are detrimental elements and should not be presentin amounts exceeding about 0.3% or, more advantageously, about 0.2% orabout 0.1%, of each.

It is found that cobalt-containing alloys, i.e., alloys containing about4% to about 12% cobalt, display improved properties from the standpointsof strength and corrosion resistance. These alloys more advantageouslycontain reduced proportions of aluminum, e.g., 3% or 3.5% or more ofaluminum, increased proportions of carbon, e.g., up to about 0.25%carbon, and increased proportions of titanium such that the titaniumcontent is at least about six times the carbon content and is up toabout 4% or 4.5%. In such alloys, the chromium content may be reduced toas low as about 15% with concomitant increase in high temperatureproperties. Such alloys are strong at 1800 F., have improvedstress-rupture strength at lower temperatures, e.g., 1350 F. to 1500"F., and have good room temperature ductility, even when cast usingcurrently common fine-grain casting techniques which involve the use ofinoculated molds. In addition, the alloys retain the long-timestructural stability which characterizes the aforementioned essentiallycobalt-free alloys. These alloys accordingly contain about 15% to about18% chromium, about 4% to about 12% cobalt, about 0.75% to about 2.2%molybdenum, about 1% to about 3% tungsten, about 0.5% to about 2%columbium, about 1% to about 3% tantalum, about 3% to about 6% or 7%aluminum, titanium in amounts at least six times the carbon content upto about 4%, about 0.1% to about 0.2% carbon, about 0.01%} to about0.04% or 0.05% boron, about 0.01% to about 0.2% zirconium, and thebalance essentially nickel. Usually, the total content of aluminum andtitanium in the alloys does not exceed about 7% or 7.5 For alloyscontaining about 10% cobalt, a preferred range comprises about 15.5 toabout 17% chromium, about 8% to about 11% cobalt, about 0.75% to about2.2% molybdenum, about 1.8% to about 3% tungsten, about 0.75% to about1.25% columbium, about 1% to abo t a a m,

about 3% to about 4% aluminum, about 3% to about 4% titanium, about0.14% to about 0.2% carbon, about 0.01% to about 0.05% boron, about0.05% to about 0.15 zirconium, and the balance essentially nickel. Foralloys containing about 5% cobalt, a preferred range comprises about15.5% to about 17% chromium, about 4% to about 6% cobalt, about 0.75 toabout 2.2% molybdenum, about 1.8% to about 3% tungsten, about 0.75% toabout 1.25% columbium, about 1% to about 2% tantalum, about 3% to about4% aluminum, about 3% to about 4% titanium, about 0.14%; to about 0.2%carbon, about 0.01 to about 0.03% boron, about 0.05 to about 0.15%zirconium, and the balance essentially nickel.

The aforementioned cobalt-containing alloys, when properly heat treated,will have a stress-rupture life of at least about 30 hours at 1800 F.and 22,000 p.s.i. stress and of at least about hours at 1350 F. and90,000 p.s.i. stress. More advantageous alloys containing nominally 4%aluminum and 3% titanium will have stressrupture lives of at least about50 hours and at least about 200 hours under the respective testconditions.

In the alloy provided in accordance with the invention, chromium andmolybdenum are carefully controlled in amount to enable obtainingimproved corrosion resistance at temperature and satisfactorystress-rupture properties as contemplated in accordance With theinvention. Thus, chromium is desirably about 16% to about 18% andmolybdenum is about 1% but does not exceed about 2.5 In some instances,chromium may be increased up to about 19% or 20% with improvement incorrosion resistance, e.g., sulfidation resistance, in instances whereinmicrostructural stability during long periods of exposure to elevatedtemperatures are of reduced importance. It is important from thestandpoint of stress-rupture life that as chromium is increased from 15%to about 20%, the molybdenum be reduced from about 2.5 to about 1% orabout 0.5%. The elements columbium, tantalum, tungsten and aluminum allcontribute in the controlled amounts employed to obtaining the highstrength developed in the alloy provided in accordance with theinvention. When any of these elements is employed in amounts eithergreat or lesser than those given hereinbefore, less satisfactory resultsfrom the standpoints of strength or ductility or both are obtained.Boron and zirconium are beneficial elements having reference to thestress-rupture properties. It is found that carbon is an importantelement in regard to the development of satisfactory stress-ruptureproperties in the alloys. For example, in one instance, an essentiallycobalt-free alloy otherwise in accordance with the invention butcontaining ony 0.01% carbon developed an unacceptable stress-rupturelife of only 10 hours at 1800 F. and 22,000 p.s.i. whereas a similaressentially cobalt-free alloy containing 0.03% carbon developed asatisfactory stress-rupture life of 40 hours at 1800 F. and 22,000p.s.i. On the other hand, when carbon exceeds about 0.08%, thestress-rupture properties of the alloys are again detrimentallyaffected, unless the appropriate adjustments in composition,particularly with regard to cobalt, aluminum and titanium notedhereinbefore, are made. Titanium may be employed in amounts notexceeding about 0.75 or, more advantageously, not more than 0.5% or even0.25%, in the special essentially cobaltfree alloys which contain notmore than about 0.08% carbon described hereinbefore. However, theaforementioned cobalt-containing alloys also having increased contentsof titanium, e.g., up to about 3.5% or 4% titanium, with reducedaluminum contents such that the total content of aluminum plus titaniumis about 6.5 to about 7.5 advantageously contain substantially increasedcarbon contents, e.g., about 0.14% to about 0.18% or about 0.2%. Suchalloys also advantageously contain about 2% molybdenum for increasedstrength, particularly at 1350 F., while maintaining freedom from theformation of brittle phases, e.g., sigma phase, on long-time heating.

h lo g a e 1 i c udes the composition of In addition, the tensile testsconducted upon cast-to-size test bars at room temperature have indicateda yield strength (0.2% offset) of about 120,000 p.s.i. along with TABLEI Percent Mo Cb eighteen alloys produced in accordance with theinvention by vacuum melting and vacuum casting. The balance of the alloyin each case is essentially nickel.

Alloy No.

elongations of the order of 3% or 5% or more for alloys within theinvention.

In order to demonstrate the improved corrosion resistance achieved inaccordance with the invention, a number of corrosion tests wereconducted wherein a specimen of the test alloy was heated in contactwith a molten mixture of 90% sodium sulfate and sodium chloride in anair atmosphere to a temperature of about 1700 F. for

a period of about four hours. It was found that the specimens of AMSS391 alloy were destroyed in the course of the test whereas specimensmade of the alloy of the present invention were not attacked. Theaforementioned cobalttaining alloys withstand the severe molten saltcorrosion test when subjected thereto for eight hours or even sixteenhours. Alloy No. 16 withstood this severe test for a period of hourswithout attack but started to show slight evidence of corrosion after200 hours. However,

40 Alloy No. 18 still showed no evidence of corrosion after 200 hours inthe test. This alloy was strong in all conditions of testing as shown inTable III and was microstructurally stable after long-time elevatedtemperature exposure, i.e., the alloy did not develop a brittle phasesuch as sigma phase. In another test conducted at 1450 F. in anoxidizing gas atmosphere containing sulfur dioxide using specimenscoated with a mixture in equal parts of sodium sulfate and magnesiumsulfate, it was found that the corrosion resistance of the alloysprovided in accordance with the invention was on the order of ten tofifty times better than that of the AMS 5391 alloy. The alloys alsoexhibit improved elevated temperature sulfidation resistance underalternating reducing and oxidizing conditions.

Castings produced in accordance with the invention may be employed inthe as-cast condition with good results. Heat treatment of the castingsmay be employed for the purpose of improving certain properties. Forexample, when it is desired to improve stress-rupture life of theaforementioned cobalt-free alloys, the castings may be subjected to asolution heat treatment comprising a heating at about 2100 F. to about2200 F., e.g., 2150 F., for a time of about one to about ten hours,e.g., about two hours. The solution heat treatment may be followed by anaging treatment at about 1600 F. to about 1700 F., e.g., about 1650 F.,for about ten hours to about fifty hours, e.g., twenty-four hours. Theaforementioned cobalt-containing alloys are heat treated by solutionheating in the temperature range of about 1925 F. to about 2075 F.,e.g., about 1975 F. to about 2050 F., for a time of about one to aboutten hours, e.g., about two to about six hours. The solution treatmentmay be followed by an aging treatment at about 1500 F. or 1550 F. toabout 1650 F. for about 24 to about 16 hours. An advantageous heattreatment comprises two beatings within 75 the temperature range ofabout 1925 F. to about 2075 fiMMMMHfiMHEHUBBBBBB 0 0 0 0 0 O 0 0 0 U 0 00 0 0 0 0 0 n ea m n m l23 8333m334 W l 0 .L n w C 11 1 a 0 u u mu m Q mm w 7 ,S m S t 1t 24 0382241313102727535874 8347764437463 4386 6 6 5 5I0 e 5555 S HF. 1111112 .11 .1111 1 55 w W 0 w m m 6445 6657 9 2 4.42234452455 0 0 H 0000 0 t w um m hhhh h mam 922225120555220552 S .55 O4446 6 n L 2. 2 .22 222 w M D 222 1 O n L L LL LLL t Pwm mmmm m m U mmmn .1 m e no .4 u :l llllllllmw ovnmllwlll 0% e W C w 31 m S u 3 1 0 0.m n es mama m n w hO h H 6551 6 0 0 0 MW f. mm HWHM H wm W 2492145236 2427 589766 M. 6436634464H m 297 5 S .1 a a a 3 0.36am m fiumfimmfi a anmm 2 that... a. m a mmaaam ma t a 636 e h d l 2 e h D. mhmmm mmamm w am. a, 0 C C n as y. as I 622111121 1121 222 m m C m m H H W F F T L .1 Lof S 0000 00 e o s 1 0 m mm O MHRM E hhhh hh E K m m m ab S 5 t 1 L 22 26 L L 0 0 2 eo B rrrrm r. B 1 1 1 .mmm15mm559m5m 7 A mtmamrm Mm A 0 m ama a r i T T m n mm uwee eos w U ln O W n mmn-ow m nummmmm mmmmmmwm m P am mmwmaem m 0 MW at P eIm m Manama o ABBCAAAABDEFGAEEEEEEEEEEDEFGEEEEEEEEEE m m m m .adddddnd II n n n N 0 C t eeee SW PH nmmh w mmfl m mh m n N n u n n u 1 t r. seeeeete mm 4 53 T HHHH H m m m m m H MS m T n 00 S a S b 1t n a n n n n u u .a .Hd H. .1 S .w N u u u E ;m S t4L 5 u Tn. u h m n. 0 6 f t u v. mm h m t m s m u u .1 "m m T.m a m CABCDEF G A L 2 5 6 6 79111111111 1 Specimen unbroken.

7 F., e.g., about 1975 F. to about 2050 F. for about two to about sixhours, with the first heating being at a higher temperature, followed byan aging treatment. For example, a four hour heating at 2050 F.,followed by a four hour heating at 1975 F., followed by an agingtreatment has been used with good results.

Castings produced from the alloys provided in accordance with theinvention may be employed not only in cast aircraft, industrial, marineand automotive gas turbine blades but also in cast stationary gasturbine components such as guide vanes, nozzle partitions and other castgas turbine components which are subjected to corrosive environments atelevated temperatures. Sulfur compounds are normally present in fuelsused for gas turbines. It is found that when gas turbine engines alsoingest salt, as is the case in marine service of various types, thatattack upon the hot surfaces of the engines is vastly accelerated. Theinvention is particularly useful for the production of parts which mustoperate in the presence of salt, e.g., sodium chloride, as encounteredin gas turbines operated at sea, including marine and aircraft gasturbines.

Castings may be produced from the alloys provided in accordance with theinvention using commercial vacuum melting and vacuum casting equipmentand employing investment molds, static casting, etc. The alloys shouldbe prepared by vacuum melting. It is also desirable to vacuum cast thealloys. In commercial practice, it is permissible to prepare remeltstock by vacuum melting and then remelt and cast under an argonatmosphere.

We claim:

1. An alloy having an improved combination of elevated temperaturestress-rupture strength and corrosion resistance consisting essentiallyof about 15% to about 18% chromium, about 8% to about 11% cobalt, about0.75% to about 2.2% molybdenum, about 1.8% to about 3% tungsten, about0.5% to about 2% columbium, about 1% to about 3% tantalum, about 3% toabout 4% aluminum, about 0.1% to about 0.2% carbon, about 3% to about 4%titanium with the total content of aluminum and titanium not exceedingabout 7.5%, about 0.01% to about 0.05% boron, about 0.01% to about 0.2%zirconium, and the balance essentially nickel.

2. An alloy in accordance with claim 1 wherein the total content ofaluminum and titanium is at least about 6.5%.

3. An alloy according to claim 1 wherein the total content of aluminumand titanium does not exceed about 4. An alloy according to claim 1wherein the carbon content is about 0.14% to about 0.2%.

5. A corrosion resistant alloy having high elevated temperature rupturestrength consisting essentially of about 16% chromium, about 10% cobalt,about 2% molybdenum, about 2.5% tungsten, about 1% columbium, about1.25% tantalum, about 4% aluminum, about 3% titanium, about 0.18%carbon, about 0.02% boron, about 01% zirconium, and the balanceessentially nickel.

6. A corrosion resistant alloy having high elevated temperature rupturestrength consisting essentially of about 16% chromium, about 10% cobalt,about 2% molybdenum, about 2.5% tungsten, about 1% columbium, about1.25% tantalum, about 3% aluminum,

' about 4% titanium, about 0.18% carbon, about 0.02%

boron, about 0.1% zirconium, and the balance essentially nickel.

References Cited UNITED STATES PATENTS RICHARD O. DEAN, Primary Examiner

